A spherical plug valve, also known as a ball valve, includes a body structure within which is movably located a plug member or ball having a through passage and defining a spherical sealing surface. The ball may have any suitable means for connecting to an operative stem to provide rotational movement of the ball. Ball valves are provided with seat members which are typically movable within the valve body and which establish sealing engagement with the spherical sealing surface of the ball and a sealing surface formed in the body, thus controlling the flow of product, such as liquids or gases, downstream of the valve when the ball is in the closed position. The seat members are of circular form having typically an internal conical configuration or an internal spherical concave configuration for sealing engagement with the spherical sealing surface of the ball and a sealing surface conforming to a sealing surface in the body.
The majority of ball valves currently used in industry are designed with two or three piece body structures, enabling body sections to be separated for ease of removing the ball and seat elements. However, two or three piece body structures are not entirely satisfactory because body leakage can occur at body seals and the valve body structure may not be capable of withstanding significant line stresses which are prevalent in piping systems controlled by such valves. Also, two or three piece body style valves require removal from the piping system for maintenance, thus increasing cost of time for maintenance and longer system shutdown time. Thus, it is desirable to be able to utilize a valve assembly that is repairable without the requirement of removing the valve from the piping system.
Typically, what is referred to as top entry type ball valves, provide the ability to accomplish such maintenance while the valve is retained in-line, thus minimizing cost of time for maintenance and system shutdown time. Top entry type ball valves have been used for a considerable period of time and are considered to minimize many of the undesirable features of ball valves having sectioned body structures. The advantages of top entry type ball valves include a one piece body structure that will withstand line stresses and with no body joints for the possibility of leakage due to body seal failure. The circular seats of ball valves establish sealing contact with the spherical surface of the ball, requiring the end faces of the seat rings to extend over the spherical surface of the ball in both closed and open positions of the ball. The ball is therefore contained within the inner circumferential sealing point of the seat rings.
Some top entry type ball valves have circular recesses incorporated within the valve body to receive the seat rings. It is difficult therefore to achieve retraction of the seat rings to a position sufficient to provide clearance for installation or extraction of the ball. Normally, this requires the need for special tools and/or additional holes through the valve body wall for insertion of special tools. These special tools are typically of the type that maintenance personnel will not ordinarily possess. Additional holes through the valve body wall require plugging and present potential hazardous leakage points to the atmosphere, which could develop due to improper plugging or due to corrosion of the plugs or wall structure adjacent the plugs. Also, manufacturing costs are significantly increased when such valve body plugs are employed. Other designs of top entry type ball valves omit the circular seat recesses in the body and require an opening of excessive diameter or size in the top of the body sufficient in size to permit installation or extraction of the seat rings and ball simultaneously. The aforementioned excessive large opening in the top of the body creates an increased effective sealing area of the bonnet seal, therefore usually requiring additional bolting to secure the bonnet to the body sufficient to withstand the internal pressure acting against the increased effective sealing area of the bonnet seal. Also, this requires additional material in the body and bonnet to develop increased costs of material and manufacturing. In certain sizes and pressure classes of ball valves with this type of construction, limitations can be realized due to the increased size of the outer configuration of the body and bonnet interfering with the end flanges integral with the body and conforming to flange face to face dimensions as specified by industry standards. These disadvantages have significantly retarded wide use of top entry type ball valves in the past.